Monocytes are phagocytic cells that belong to the mononuclear phagocyte system and reside in the circulation. These cells originate in the bone marrow and remain only a short time in the marrow compartment once they differentiate. They then enter the circulation and can remain there for a relatively long period of time, e.g., a few days. Monocytes can enter the tissues and body cavities by a process known as diapedesis, where they differentiate into macrophages and possibly into dendritic cells. In an inflammatory response, the number of monocytes in the circulation may double, and many of the increased number of monocytes diapedese to the site of inflammation. For a review of monocytes and their functions, see, e.g., Gallin, et al. (eds), 1988, Inflammation: Basic Principles and Clinical Correlates, Raven Press, N.Y.; van Furth (ed), 1985, Mononuclear Phagocytes: Characteristics, Physiology and Function, Martinus Nijhoff, Dordrecht, Netherlands.
Antigen presentation refers to the cellular events in which a proteinaceous antigen is taken up, processed by antigen presenting cells (APC), and then recognized to initiate an immune response. The most active antigen presenting cells have been characterized as the macrophages, which are direct developmental products from monocytes; dendritic cells; and certain B cells.
Macrophages are found in most tissues and are highly active in internalization of a wide variety of protein antigens and microorganisms. They have a highly developed endocytic activity, and secrete many products important in the initiation of an immune response. For this reason, it is believed that many genes expressed by monocytes or induced by monocyte activation are important in antigen uptake, processing, presentation, or regulation of the resulting immune response.
Despite the importance of monocytes to immune system function, these cells remain poorly characterized, both in terms of the proteins they express and in terms of many of their functions, in particular, the processes and mechanisms related to the initiation of an immune response, including antigen processing and presentation. There is thus a need in the art for agents useful in the diagnosis and treatment of medical conditions caused by, e.g., the inappropriate regulation, development, or physiology of antigen presenting cells.